Artificial spinal disk

ABSTRACT

An artificial spinal disk comprises a central capsule that is configured to slide laterally within the disk space with one or more of flexion, extension, and lateral bending of the spine so as to shift an instantaneous center of rotation of the artificial disk. In one embodiment, the invention comprises an artificial spinal disk comprising a first plate having an inwardly directed surface, a second plate having an inwardly directed surface facing generally toward the inwardly directed surface of the first plate, and a central capsule with outwardly directed opposed faces that slidably mate with the inwardly directed surfaces of the first and second plates.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 10/838,515. Thisapplication claims priority under 35 U.S.C. Section 119(e) to U.S.Provisional Applications 60/467,655 filed on May 2, 2003 and 60/513,238filed Oct. 21, 2003. The disclosures of all of these applications arehereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

A wide variety of artificial spinal disk designs have been developedover the past several years. Some designs, such as those described inU.S. Pat. Nos. 6,001,130 and 5,123,926 include resilient plastic orfluid filled bag type structures that are placed between adjacentvertebra. These designs provide flexibility, but present the risk ofrupture or breakage, and can be difficult to contain effectively withinthe disk space. Other designs have attempted to use ball-and socket typecouplers between endplates or other retaining devices attached to thevertebral bodies. Currently, devices which use metal-metal interfacesrather than resilient bodies are favored for their reliability andstrength. However, these types of couplings do a poor job of imitatingthe natural relative movement of vertebral bodies separated by a naturalanatomical disk. Furthermore, this type of replacement disk typicallyfocuses all the forces from weight and motion in a single direction andon a very small part of each vertebral body. This can cause excessivestress on the bone in the area where the artificial disk connects to thevertebral body. Improved designs that reduce these problems are neededin the art.

SUMMARY OF THE INVENTION

An artificial spinal disk comprises a central capsule that is configuredto slide laterally within the disk space with one or more of flexion,extension, and lateral bending of the spine so as to shift aninstantaneous center of rotation of the artificial disk. In oneembodiment, the invention comprises an artificial spinal disk comprisinga first plate having an inwardly directed surface, a second plate havingan inwardly directed surface facing generally toward the inwardlydirected surface of the first plate, and a central capsule withoutwardly directed opposed faces that slidably mate with the inwardlydirected surfaces of the first and second plates.

In another embodiment, an artificial spinal disk comprises a pluralityof separate pieces, wherein the separate pieces are configured and sizedto be placed in the disk space separate from one another. The piecescomprise couplers such that the separate pieces are attached to form acompleted artificial disk only after installation within the disk space.In one such embodiment, the separate pieces of the artificial diskcomprise at least first and second bone plates and a central capsule.

Methods of spine surgery are also provided. In one embodiment, a methodof spine surgery comprises placing a first portion of an artificial diskinto a disk space; and

separately placing one or more additional portions of the artificialdisk into the disk space and mechanically coupling the additionalportions to one or more portions previously placed inside the disk spaceso as to assemble a complete artificial disk within the disk space fromartificial disk pieces that are separate outside of the disk space.

In another embodiment of the invention, a surgical kit for spinalsurgery comprises a first bone plate configured for attachment to afirst vertebral body;

a second bone plate configured for attachment to a second vertebralbody; and

a central capsule configured to couple to the first bone plate and thesecond bone plate;

wherein the first bone plate, the second bone plate, and the centralcapsule are uncoupled from one another to allow for separateinstallation in a disk space during spine surgery. In some embodiments,the first bone plate and the second bone plate comprise a plurality ofuncoupled segments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a first embodiment of an artificial spinal diskwith a sliding capsule.

FIG. 2 is a side view of a second embodiment of an artificial spinaldisk with a sliding capsule.

FIG. 3 is a cross sectional view of a specific embodiment of the slidingcapsule design of FIG. 1.

FIG. 4 is an exploded view of the disk of FIG. 3.

FIG. 5 is a cross sectional view of a specific embodiment of the slidingcapsule design of FIG. 2.

FIG. 6 is a perspective view of a multi-piece bone plate for couplingthe sliding capsules to upper and lower vertebral bodies.

FIGS. 7A and 7B are top and side views of a specific embodiment of abottom bone plate.

FIGS. 8A and 8B are top and side views of a specific embodiment of a topbone plate.

FIG. 9 is a side view of the capsule of FIG. 3 (in cross section)coupled to the top and bottom bone plates of FIGS. 7 and 8.

FIG. 10 illustrates an alternative artificial disk embodiment.

FIG. 11 illustrates a three-piece bone plate coupling the slidingcapsules to upper and lower vertebral bodies.

FIG. 12 is a side cutaway view of the artificial disk embodiment of FIG.10.

DETAILED DESCRIPTION

One embodiment of an artificial disk in accordance with the invention isshown in FIGS. 1A, 1B and 1C. In this embodiment, endplates 12, 14sandwich a sliding central capsule 18. As shown in these Figures, afirst plate 12 defines a first inwardly directed surface 13 and thesecond plate 14 defines a second inwardly directed surface 15. A centralcapsule 18 defines opposed outwardly directed surfaces 17, 19 thatslidably mate with the inwardly directed surfaces of the plates. Thus,the central capsule can slide toward and away from opposed edge portionsof the endplates as the relative endplate orientation changes duringflexion, extension or lateral bending of the motion segment in which theartificial disk is installed. With this design, the endplates 12, 14 canalso rotate with respect to one another as the central capsule slidesbetween them. In this embodiment, the endplates have convex sphericalcontours, and the central capsule 18 is generally cylindrical with topand bottom surfaces that are concave spherical contours mating with theconvex spherical contours on the endplates. Thus, the central capsule 18is generally cylindrical with a hour-glass shaped cross-section.

Another embodiment of an artificial disk with a central sliding capsuleis illustrated in FIGS. 2A, 2B, and 2C. In this embodiment, the innersurfaces 13, 15 of the endplates 12, 14 are concave rather than convex.The capsule 18 includes mating convex surfaces 17, 19 that slides alongthe endplate surfaces in a manner analogous to that shown in FIGS. 1A,1B, and 1C.

There are a variety of important benefits of such a sliding capsule 18.One is that the instantaneous center of rotation of the motion segmentis allowed to move around inside the disk space with the capsule duringlateral bending, flexion, and extension. Also, the central capsulespreads mechanical stresses over a larger portion of the endplates andthus over the adjacent vertebral bodies as well. This mimics the naturalbehavior of a spinal disk much better than existing artificial diskdesigns. Also, this leads to a reduced tendency for migration andloosening following installation, since stresses due to spinal movementsare not continually focused in the same direction or location.

One mechanical design for implementing the above described slidingcapsule is shown in FIGS. 3 and 4. Referring now to these figures,retainer disks 22A and 22B are secured to respective end plates 14 and12 on pedestals 24A and 24B with screws (designated 26A, B, C, and D inFIG. 4). The retainer disks also have a spherical contour substantiallymatching that of the endplates. The pedestals 24A, 24B may be capturedin the end plates in recesses or they may be integral with the endplates as shown in FIG. 3. Each retainer disk is secured tightly to thepedestal with the screws such that it does not move with respect to theendplate. Because of the pedestal, however, the underside of eachretainer disk is raised up off of the inside spherically contouredsurface of the endplate by the height of the pedestal. In an alternativeembodiment, a single screws can be used, or a single screw can be madeintegral with their respective retainer disks 22A, 22B, with theretainer disks held away from the surfaces of the plates without thepedestals by screwing the threaded shafts down to a stop in the plate,for example. If desired, a ring clip or other device could be used tofix unthreaded shafts for the retainer disks 22A, 22B, such that thedisks themselves are fixed away from the surfaces of the plates but areallowed to rotate about their central axes.

Captured underneath each endplate, between the surface of the endplateand the underside of each retainer disk, are sliding inner disks 30A and30B, which are also spherically contoured to match the contour of theendplate inner surfaces. The thickness of these sliding disks 30A, 30Bis selected with respect to the height of the pedestals 24A, 24B suchthat each disk 30A and 30B are slidably captured between the innersurface of the endplate and the underside of the respective retainerdisk. The two separate endplates, with attached sliding and retainerdisks, are held in facing relation by a sliding inner disk clamp, whichin this embodiment comprises two parts, designated 40A and 40B in theseFigures, and which are held to each other with screws. The clamp pieces40A and 40B engage the edges of the sliding disks 30A and 30B in atongue and groove arrangement. In the pictured embodiment, the edge ofeach sliding inner disk 30A and 30B is provided with a groove 42A and42B, and the inner surface of the clamp is provided with a pair ofextending flanges 44A and 44B. When the flanges on the clamp engage thegrooves of the sliding disks 30A and 30B, a cylindrical sliding assemblywith an hourglass shaped cross section is created comprising the clamp40A, 40B and the sliding disks 30A, 30B. This sliding assembly couplesthe endplates via the position of the sliding disks under the retainerdisks and is slidable with respect thereto between the endplates and theretainer disks 22A and 22B.

The amount of lateral motion and rotation that the sliding assembly isallowed is governed by the shape and size of central openings in thesliding disks with respect to the shape and size of the pedestals 24Aand 24B fixed to the center of the endplates. The sliding disks will beable to slide away from the center and rotate until the edges of theopenings in the sliding disks contact the sides of the pedestals. In oneembodiment, it has been found advantageous for the relative dimensionsof these features to allow for a few millimeters of lateral movement.For round pedestals and openings, rotation around the central axis ofthe device is unlimited throughout 360 degrees. It has been foundadvantageous, however, to use the oblong shapes shown in FIG. 4, whichlimit rotation to about 30-60 degrees.

An alternative embodiment is illustrated in cross section in FIG. 5.This embodiment corresponds to the capsule design illustrated in generalin FIG. 2. In this design, the retainer disks 22A and 22B and the innersliding disks 30A and 30B are curved in the opposite contour from theembodiment of FIGS. 1 and 3-4. Thus, in the embodiment of FIG. 5, theconcave sides of the retainer disks and sliding disks face each other,and the convex sides face corresponding concave surfaces of theendplates 12, 14. Operation of this embodiment is analogous to thatdescribed above, with the inner sliding disks 30A and 30B sliding alongthe endplate inner surfaces and between the retainer disks and theendplate surfaces during flexion, extension, and lateral bending of thespinal column.

One advantage of the design of FIG. 5 is that the clamp 40A, 40B ofFIGS. 3 and 4 can be eliminated. This can be accomplished by includingmating press-fit flanges 46A, 46B around the outer edges of the twosliding disks 30A, 30B. To assemble the device, each half is constructedcomprising an endplate, a retainer disk, and a sliding inner disk. Then,the two halves are coupled with a snap fit that engages the flanges 46A,46B and holds the two halves together.

All components of the device may be made of biocompatible metals andmetal alloys such as stainless steel or titanium. In one embodiment, thesliding coefficient of friction between the disks and the endplatesurfaces is reduced by coating the sliding surfaces with a low frictioncoating. One example of such a useful coating is known as Casidiam™diamond-like carbon coating. This coating typically includes carbon,hydrogen, and possibly some additional dopant materials and is a mixtureof tertagonal diamond type carbon crystal structure and trigonalgraphitic carbon crystal structure. It is a commercially availablecoating and is used in a variety of industrial and medical applicationsrequiring hardness, chemical inertness, biocompatibility, and lowfriction.

The device may be installed in a variety of ways. The device may, forexample, be installed in an anterior surgical procedure usinginstallation and securement methods currently used for artificial disksof conventional design. For example, the endplates 12, 14 could includevertically extending central fins to engage the vertebral bodies oneither side of the disk. This installation technique, however, hasserious drawbacks. First, anterior installation is inherently risky dueto the presence of the large blood vessels that run down the anterior ofthe spinal column. These vessels are especially vulnerable in the eventthe artificial disk needs to be removed, as revision surgeries mustcontend with scar tissue and adhesions that form in the surgical fieldand attach to these vessels. It is therefore desirable to provide anartificial disk design that is installed via a posterior orposterior-lateral approach. Although beneficial from a surgical point ofview, the spinal cord, facets, lamina, and other bony structures in theposterior of the spine limit the available insertion space. Thisdifficulty has limited the availability of posterior inserted artificialdisks. To resolve this difficulty, and to increase the use of minimallyinvasive procedures, an especially advantageous embodiment has beendesigned in which the artificial disk is placed inside the disk space inseveral separate individual smaller pieces and is assembled within thedisk space.

In one such embodiment a pair of bone plates, each of which comes in twopieces, are installed and fixed to the upper and lower vertebral bodies.The bone plates include aligned channels into which a cassettecomprising the central capsule 18 plus the two endplates 12, 14 isinserted. The artificial disk thus comes in a five-piece assembly thatis inserted into the disk space one piece at a time, allowing for asmaller incision and surgical field and making posterior installation ofan artificial disk a practical surgical alternative.

Bone plates which may be used in one such embodiment are illustrated inFIGS. 6 through 8. FIG. 6 is a general conceptual 3-D view from above anupper bone plate. FIGS. 7A-B and 8A-B show plan views and side views ofupper and lower bone plates according to one embodiment of theinvention.

In this embodiment, each bone plate includes a larger section 54 and asmaller section 52. The two sections are coupled together by a tongueand groove mating region 58. In the embodiment of FIG. 6, the largersection 54 includes a dovetail groove on a straight interior edge, andthe smaller section includes a mating dovetail flange on an adjacentstraight interior edge. The two pieces 52, 54 are further held togetherwith a screw 60 (FIGS. 7 and 8) that is installed into a countersunkhole in the smaller section 52 and which terminates in a threaded holein the larger section 54. When installed, this screw holds the two parts52, 54 firmly together such that relative motion along the groovedmating region 58 is prevented after the pieces are installed.

The bone plates may also incorporate captured pins 72 that are deployedinto the vertebral body after installation. A variety of pin deploymentmethods are known and could be used, including those described in U.S.Pat. Nos. 5,800,547; 5,123,926; and 5,102,950, all three of which arehereby incorporated by reference in their entireties.

When mated as shown in FIGS. 7A and 8A, the two sections 52 and 54create a dovetail channel 70 that extends diagonally on the surface ofeach bone plate. As described briefly above, the channels 70 acceptmating dovetail flanges which extend from the outer surfaces of theendplates 12, 14 of FIGS. 1 to 3. In this way, the cassette comprisingthe end-plates and sliding capsule can itself be slid into positionbetween the bone plates after installation of the bone plates.

In one embodiment, disk installation proceeds as follows. A lateralposterior hemilaminotomy incision is made and the natural disk isresected in a conventional manner. For the bone plate design shown inFIGS. 6-8, this incision would be on the left of the spine. Afterremoval of the natural disk, the larger of the two bone plate sections52 for either the top or bottom vertebral body is inserted through theincision. This section of the bone plate is then pushed laterally overto the right side of the disk space into alignment with the right sideof the vertebral body and such that the groove on its flat interior edgeruns substantially straight from back to front. The captured pins 72 arenow deployed. The most convenient method is typically the insertion ofan expandable device that presses the pins into place in the facing bonetissue. After the larger section 54 is installed, the smaller section 52is installed by sliding it straight into the left side of the disk spacesuch that its dovetail flange is engaged with the dovetail groove in thefirst section 54 and such that the channel 70 is aligned across theentire bone plate. The pins of the second installed section 52 are thendeployed in the same manner as the first. This same procedure isrepeated to attach a bone plate to the other vertebral body. To ensurethat the channels 70 in both bone plates are appropriately aligned alongtheir length and with each other, it is possible to use a tool that canslide into both channels to test that they do not diverge in position ordirection from back left to front right along the bone plate surfaces.

After the bone plates of FIGS. 7 and 8 are installed, the cassettecomprising the endplates 12, 14, and central sliding capsule 18 isinserted by mating a dovetail flange on the outer surfaces of theendplates 12/14 (not shown in FIGS. 3 and 4) to the dovetail grooves 70and sliding the cassette into position between the bone plates andapproximately the center of the disk space. The cassette can be held inplace with a set screw 80. A conceptual side view of an assembly ispictured in FIG. 9.

An embodiment having a central capsule similar to that described abovewith reference to FIGS. 2 and 5 is illustrated in FIGS. 10-12. As withthe embodiment of FIG. 5, sliding disks 30A, 30B slide between theendplate inner surfaces and the bottom surfaces of inner retaining disks22A, 22B. The retaining disks, in this embodiment, comprise threadedshafts that are screwed into threaded through holes 82A, 82B in the endplates 12, 14. After they are threaded down to the appropriateclearance, the bottom of the shafts are orbited in place to lock themtightly and prevent any rotation or movement out of position. In thisembodiment, the endplates 12, 14 are captured in between the bone platesthrough central orifices 92, 94. Grooved posts 96A, 96B extend from eachendplate 14, 12 respectively. Captured in each grooved post is a snapring. When the posts are engaged in the bone plates, ears on each snapring engage an internal groove 102 in the orifice of each bone plate.

As shown in FIG. 11, the endplates may come in three parts instead oftwo as shown in FIGS. 6-8. The rightmost portion 104 may be installedfirst, followed by the central portion 106, and then the left-mostportion 108. The portions are engaged by a series of mating dovetailtongue and groove segments. In the embodiment of FIG. 11, the centralportion 106 has groove segments on the left, and tongue segments on theright. With this design, a tongue and groove mating along about half ofthe length of the bone plates can be made during installation with asliding motion of only the length of a single dovetail segment. Ifdesired, lips above and/or below the segments can be provided todiscourage bone growth between the segments after installation.

FIG. 12 illustrates the embodiment of FIG. 10 after complete assembly.With this embodiment, the centerline of the device moves in accordancewith the human body in relationship to the centerline, mimicking theresponse of a natural disk.

To install the device in the spinal column, the bone plates areinstalled as shown in FIG. 11. The bone plates may include deployablespikes or pins as described above, or they may have integralpre-deployed pins on their outer surfaces that are pressed into thevertebral bodies during installation.

The vertebra are then distracted to allow the central cassettecomprising endplates 12, 14, central sliding capsule to be insertedbetween them. The vertebra are distracted to allow clearance for theposts 96A, 96B before they are set in the orifices 92, 94 in the boneplates. Once the posts are aligned with the orifices, the distraction isremoved, and the posts drop into the orifices, engaging each snap ring98A, 98B in its respective groove in the bone plate. The snap rings maybe dimensioned to deform slightly during installation and snap intoplace, or a tool can be used to compress the rings slightly and allowthe posts to engage the orifices. Toll access holes 106, 108 can beprovided for this purpose, and to compress the rings for cassetteremoval, should removal be necessary.

The facets can also be addressed at the same time the artificial disk isbeing placed, and attention to the spinous process abutment can also beaddressed at the time of surgery. In some surgical procedures, theposterior elements will also have an implant applied to the facets toimprove on range of motion in flexion and extension without pain. Thesefacet implants or articulations will facilitate the gliding mechanismthat is well documented on scinradiography when the spine is takenthrough a range of motion in flexion, extension, lateral bend andtorque. If the facet joint is not addressed, which is a significantstabilization unit of the motion segment, there may continue to beproblems with back pain. At the time of our artificial diskimplantation, the capsule of the facet joint may be removed, and a metalon metal artificial facet may be inserted to minimize pain and topreserve movement.

The facet arthroplasty will be an articulation with the inferolateralfacet and superomedial facet. This arthroplasty will have a mechanismthat will allow flexion, extension and lateral translation to occur.This arthroplasty may be accomplished by opening the facet joint andplacing the implants on the articular cartilage (as illustrated in FIG.2 for example). In may cases there may hypertrophy or arthrosis to thesecomplexes, which may require a partial resection with a high speed burror osteotome. Once the opening in the facet joint is achieved thearthroplasty can then be undertaken, and the implants are then attachedto the inferior aspect of the ventral surface of the vertebra above andto the dorsal articulating surface of the vertebra below. With thisfacet arthroplasty done bilaterally, which can be achieved throughminimally invasive technology, and the artificial disk in place, we havenow addressed the three articulations, anterior, middle column andposterior.

The spinous processes can be partially resected to give space if thereis abutment noted. A space may be created between the spinous processesto allow a placement of a shield with a metal on metal articulation atthe spinal laminar junction of the vertebra above and below. This metalon metal articulation will give some partial support and also preventthe abutment of spinous processes which would restrict range of motionand could result in pain. This will not only give a partial ligamentousstability, but will also keep the spinous processes from abutting. Asyou can see, our artificial disk complex comprises both a posteriorplacement or lateral placement of the disk with supplementing the facetand possibly the spinous processes. Therefore, the entire complexanterior, middle and posterior column can be addressed to preservecircumferential stability to the motion segment. The artificial diskembodiments described herein do not preclude the device from beingplaced anteriorly, but it may often be preferable to perform oneincision that can address both the posterior elements, as well as theinterbody disk level with that one incision.

This modular design has a variety of advantages. One advantage alreadymentioned is that the design makes a posterior surgical approachpractical. The bone plates for the vertebral bodies are inserted inmultiple pieces. As shown in FIG. 9, the cassette itself can be madesmaller than the bone plates, allowing insertion through the same smallposterior lateral hemilaminotomy incision following the two-piece boneplates.

Another advantage to this design is that it allows the artificial diskto be easily replaced with a fusion cage if this becomes necessary. Insuch a revision surgery, the artificial disk cassette can be pulled out,and replaced with another cassette comprising a fusion cage filled withharvested bone. In some embodiments, the cassette could include anattachment point for a slap-hammer so that the cassette could be removedmore easily. This process is much simpler and less traumatic thancurrent artificial disk removal procedures. With conventional anteriorinstallations, implant removal to perform a fusion often involvessignificant bone removal from the vertebral bodies to get the implantout.

The modular design described above can even be useful as a replacementfor a removed disk as well. Because the bone plates are separate fromthe central cassette, the bone plates can be made in varyingthicknesses, or two or more can be stacked, so that if bone removal fromthe vertebral bodies has significantly extended the height of the diskspace, this can be compensated for by extended bone plate thickness.Thus, during revision surgeries, the bone plates can be exchanged fordifferent versions having alternative thicknesses and sizes.

To further produce an easy and successful transition from artificialdisk to fusion, the bone plates can be made fenestrated. In someembodiments it might be desirable to replace solid plates withfenestrated ones during the revision surgery to convert from anartificial disk to a fusion. As another alternative, fenestrated boneplates could be removably attached to solid covers that are left inplace when used with an artificial disk installation but which areremoved during a revision to a fusion.

1-6. (canceled)
 7. An artificial spinal disk comprising: a first platedefining a first curved surface; a first retainer disk affixed to saidpedestal having a contour substantially matched to said first curvedsurface and held in spaced relation to said first curved surface; asecond plate defining a second curved surface; a second retainer diskaffixed to said second pedestal having a contour substantially matchedto said second curved surface and held in spaced relation to said secondcurved surface; a first sliding disk captured between said firstretainer disk and said first curved surface; a second sliding diskcaptured between said second retainer disk and said second curvedsurface, wherein said second sliding disk is coupled to said firstsliding disk.
 8. The artificial disk of claim 7, wherein said first andsecond sliding disks are coupled along their respective edges.
 9. Theartificial disk of claim 8, wherein said first and second sliding disksare coupled along their respective edges by a clamp. 10-20. (canceled)21. The artificial disk of claim 7, wherein said first and secondretainer disks and said first and second sliding disks comprise surfacescoated with diamond like carbon.
 22. The artificial disk of claim 7,wherein said first and second plates comprise concave surfaces.